An Analysis of Co Production in Cometary Comae: Contributions From Gas-Phase Phenomena

Pierce, Donna M.; A’Hearn, Michael F.

doi:10.1088/0004-637x/718/1/340

Cited by 12 publications

(7 citation statements)

References 63 publications

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“…91,101,211 In heavily irradiated disk atmospheres many species will exist in excited (ro-)vibrational states, which may then react differently with other species and require addition of state-to-state processes in the models. 287 In the outer, cold disk regions addition of nuclear-spin-dependent chemical reactions involving ortho-and para-states of key species is required. Last but not least, a better understanding of surface processes, including non-thermal desorption, chemisorption, high-energy processing of ices, diffusion through the ice mantle, and related factors have to be considered.…”

Section: Status Of Chemical Models For Protoplanetary Disksmentioning

confidence: 99%

Chemistry in Protoplanetary Disks

2013

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Abstract. The most intriguing question related to the chemical evolution of protoplanetary disks is the genesis of pre-biotic organic molecules in the planet-forming zone. In this contribution we briefly review current observational knowledge of physical structure and chemical composition of disks and discuss whether organic molecules can be present in large amounts at the verge of planet formation. We predict that some molecules, including CO-bearing species such as H2CO, can be underabundant in inner regions of accreting protoplanetary disks around low-mass stars due to the high-energy stellar radiation and chemical processing on dust grain surfaces. These theoretical predictions are further compared with high-resolution observational data and the limitations of current models are discussed.

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Section: Status Of Chemical Models For Protoplanetary Disksmentioning

confidence: 99%

Chemistry in Protoplanetary Disks

2013

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“…The X-ray-driven ionization and dissociation of molecules other than H 2 are poorly understood, but important for disk chemistry (e.g., Glassgold et al 2009). In the upper disk regions, upon gas-phase or surface recombination or due to ionization/dissociation an excess of energy may translate into (ro-)vibrational excitation of a product molecule, which may react differently with other species (e.g., Pierce & A'Hearn 2010). This aspect has so far been almost completely neglected in astrochemical models.…”

Section: Complex Organic Moleculesmentioning

confidence: 99%

Chemical Evolution of Turbulent Protoplanetary Disks and the Solar Nebula

Semenov

Wiebe

2011

ApJS

175

224

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This is the second paper in a series where we study the influence of transport processes on the chemical evolution of protoplanetary disks. Our analysis is based on a 1+1D flared α-model of a ∼ 5 Myr DM Tau-like system, coupled to a large gas-grain chemical network. To account for production of complex molecules, the chemical network is supplied with an extended set of surface reactions and photo-processes in ice mantles. Our chemo-dynamical disk model covers a wide range of radii, 10-800 AU (from a Jovian planet-forming zone to the outer disk edge). Turbulent transport of gases and ices is implicitly modeled in full 2D along with the time-dependent chemistry, using the mixing-length approximation. Two regimes are considered, with high and low efficiency of turbulent mixing. The results of the chemical model with suppressed turbulent diffusion are close to those from the laminar model, but not completely. A simple analysis for the laminar chemical model to highlight potential sensitivity of a molecule to transport processes is performed. It is shown that the higher the ratio of the characteristic chemical timescale to the turbulent transport timescale for a given molecule, the higher the probability that its column density will be affected by diffusion. We find that turbulent transport enhances abundances and column densities of many gas-phase species and ices, particularly, complex ones. For such species a chemical steady-state is not reached due to long timescales associated with evaporation and surface photoprocessing and recombination (t 10 5 years). When a grain with an icy mantle is transported from a cold disk midplane into a warm intermediate/inner region, heavy radicals become mobile on the surface, enriching the mantle with complex ices, which are eventually released into the gas phase. The influence of turbulent mixing on disk chemistry is more pronounced in the inner, planet-forming disk region where gradients of temperature and high-energy radiation intensities are steeper than in the outer region. In contrast, simple radicals and molecular ions, which chemical evolution is fast and proceeds solely in the gas phase, are not much affected by dynamics. All molecules are divided into three groups according to the sensitivity of their column densities to the turbulent diffusion. The molecules that are unresponsive to transport include, e.g., C 2 H, C + , CH 4 , CN, CO, HCN, HNC, H 2 CO, OH, as well as water and ammonia ice. Their column densities computed with the laminar and 2D-mixing model differ by a factor of 2 − 5 ("steadfast" species). Molecules which vertical column densities in the laminar and dynamical models differ by up to 2 order of magnitude include, e.g., C 2 H 2 , some carbon chains, CS, H 2 CS, H 2 O, HCO + , HCOOH, HNCO, N 2 H + , NH 3 , CO ice, H 2 CO ice, CH 3 OH ice, and electrons ("sensitive" species). Molecules which column densities are altered by diffusion by more than 2 orders of magnitude include, e.g., C 2 S, C 3 S, C 6 H 6 , CO 2 , O 2 , SiO, SO, SO 2 , long carbon chain ices,...

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“…A significant (if not well-quantified) fraction of cometary CO may come from a "distributed" coma source that might include refractory material, or from a complex network of chemical (and photo-chemical) reactions (Pierce & A'Hearn 2010), and might behave differently. The CO abundance varies across the comet population by as much as a factor of 40 (Mumma et al 2003;Bockelée-Morvan et al 2004) and as yet does not seem to be wellcorrelated with other species.…”

Section: The Carbon Abundancementioning

confidence: 99%

INFRARED SPECTROSCOPY OF COMET 73P/SCHWASSMANN-WACHMANN 3 USING THESPITZER SPACE TELESCOPE

Sitko

Lisse

Kelley

et al. 2011

The Astronomical Journal

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We have used the Spitzer Space Telescope Infrared Spectrograph (IRS) to observe the 5-37 µm thermal emission of comet 73P/Schwassmann-Wachmann 3 (SW3), components B and C. We obtained low spectral resolution (R∼100) data over the entire wavelength interval, along with images at 16 and 22 µm. These observations provided an unprecedented opportunity to study nearly pristine material from the surface and what was until recently the interior of an ecliptic comet -cometary surface having experienced only two prior perihelion passages, and including material that was totally fresh. The spectra were modeled using a variety of mineral types including both amorphous and crystalline components. We find that the degree of silicate crystallinity, ∼35%, is somewhat lower than most other comets with strong emission features, while its abundance of amorphous carbon is higher. Both suggest that SW3 is among the most chemically primitive solar system objects yet studied in detail, and that it formed earlier or farther from the sun than the bulk of the comets studied so far. The similar dust compositions of the two fragments suggests that these are not mineralogically heterogeneous, but rather uniform throughout their volumes. The best-fit particle size distribution for SW3B has a form dn/da∼a −3.5 , close to that expected for dust in collisional equilibrium, while that for SW3C has dn/da∼a −4.0 , as seen mostly in active comets with strong directed jets such as C/1995 O1 Hale-Bopp. The total mass of dust in the comae plus nearby tail, extrapolated from to the field of view of the IRS peakup image arrays, is 3-5 x 10 8 kg for B and 7-9 x 10 8 1 Guest observer, Spitzer Space Telescope -3kg for C. Atomic abundances derived from the spectral models indicates a depletion of O compared to solar photospheric values, despite the inclusion of water ice and gas in the models. Atomic C may be solar or slightly sub-solar, but its abundance is complicated by the potential contribution of spectrally featureless mineral species to the portion of the spectra most sensitive to the derication of the C abundance. We find a relatively high bolometric albedo, ∼0.13 for the dust, considering the large amount of dark carbonaceous material, but consistent with the presence of abundant small particles and strong emission features.

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An Analysis of Co Production in Cometary Comae: Contributions From Gas-Phase Phenomena

Cited by 12 publications

References 63 publications

Chemistry in Protoplanetary Disks

Chemistry in Protoplanetary Disks

Chemical Evolution of Turbulent Protoplanetary Disks and the Solar Nebula

INFRARED SPECTROSCOPY OF COMET 73P/SCHWASSMANN-WACHMANN 3 USING THESPITZER SPACE TELESCOPE

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